1. Field of the Invention
This invention generally relates to display devices and their components. More particularly, the present invention relates to spatial light modulators for use in holographic display systems, optical information processing systems, direct-write lithography systems and adaptive optics.
2. Description of the Related Art
A spatial light modulator (SLM) is an optical device that allows modulation of incoming incident light with desired amplitude or phase pattern. Spatial light modulators are commonly used in projection television and display systems. Two common types of spatial light modulators used presently are digital micro-mirror device (DMD) and liquid crystal devices (LCD). Various types of DMDs are disclosed for example in U.S. Pat. No. 4,956,619, and U.S. Pat. No. 5,083,857. A digital micro-mirror device generally comprises a matrix of micro-mirrors suspended above a substrate of the device. A voltage is applied between the micro-mirror and electrode which allows individual adjustment of the light reflection angles of the micro-mirror, and thus control of the light reflection of the array is achieved.
Liquid crystal devices (LCD) typically comprise electrodes on the opposite sides of a layer of liquid crystal that changes polarization of incident light in response to an applied voltage. When no electric field is applied, light does not changes polarization and passes through the polarization filter; when an electric field is applied, the polarization changes and the polarization filter absorbs light.
These two types of spatial light modulators, DMD and LCD, are suitable and widely utilized for projection television and computer display systems. However, these spatial light modulators have technical and physical limitations that limit their usage in holographic systems and other modern technical applications that require high resolution, i.e. small size of pixels. The DMDs and LCDs typically have a geometric size of pixel elements on the order of dozens of microns. For example, a typical micro-mirror element has size of 10-20 microns (xcexcm). Another disadvantage of existing spatial light modulators is that they have a long time of modification of the optical elements state, i.e. changing the image pattern, on the order of milliseconds.
There are other types of spatial light modulators used for specific applications. One such type is known as multiple quantum-well device that is comprised of multiple layers of semiconductor material sandwiched between transparent electrodes. The voltage applied to electrodes alters the absorption of light in the wavelength near the band gap of the material, which is typically GaAs or AlGaAs. An example of a multiple quantum well spatial light modulator, specifically for optical pattern recognition, is disclosed in U.S. Pat. No. 5,844,709. That device is designed to provide a combination of optically and electrically addressed spatial light modulators for the specific task of high performance pattern recognition. However, multiple quantum well devices are not well suited for other tasks of spatial light modulators because those devices can operate only in a limited spectrum of light near the band gap of the material, normally in the infrared spectrum. Further, the quantum-well modulator is difficult, and therefore costly, to fabricate with a large size of image area. Another type of spatial light modulator uses a magneto-optic material that changes polarization of incident light depending on the magnetization of pixel elements.
A further known type of light modulators is based on electro-optic materials that change their refractive index in response to an electric field. A first-order (linear) electro-optic effect, known as the Pockels effect, and a second-order (quadratic) electro-optic effect, known as the Kerr effect, occur in response to the electric field. Electro-optical devices typically utilize optical materials such as LiNiO3 or lead zirconate titanate ceramic (PLZT). Such electro-optic light modulators are typically used for time-modulation of a light beam. But such devices are usually not designed and not suitable for spatial modulation of light specified by a 2-dimensional high-resolution pattern. Further, the size of a typical electro-optical modulator is quite big, so it is insufficient to serve as an element of a high-resolution spatial light modulator.
A classical electro-optical modulator is comprised of a wafer or volume of electro-optic material with electrodes attached to opposite sides of the material. Such device is capable of time-modulation of light beam. There have been attempts to make a compound optical system by combining multiple electro-optical modulators on one wafer. For example, U.S. Pat. No. 4,746,942, suggests a wafer of electro-optic material with a number of surface-mounted electrodes to form multiple independently operated electro-optic modulators. However, this type of device suffers from interference of the electric fields between electrodes, which limits how many modulators can be on one die. One solution to this problem is disclosed in U.S. Pat. No. 6,486,996, which suggests an optical system comprising a plurality of discrete protrusions of electro-optic material. Each discrete protrusion is electrically and optically isolated from each other. Such device permits multiple electro-optical modulators placed closely on one die. Nevertheless, that device is designed for time-modulation of a fixed number of light beams, such as used in printers and telecommunication. That device is not designed and not capable to perform at high-resolutions because the size of the optical elements (protrusions in electro-optic material) is far too large for displaying a high-resolution image pattern.
Holography is the application of optical technology best known for reproducing three-dimensional images. In addition to the simple production of 3-dimensional images, modern holography is used in broad range of applications, which includes, for example, holographic interferometers, optical memory systems, optical information processing systems, adaptive optics and may other technical areas. Typical holographic devices utilize film for recording the interference pattern of two beams of light, one reflected from the object and one reference beam. The two beams are created from a laser passed through beam expander optics and through a beam splitter that separates incident laser light into two beams. One fraction of the laser light is reflected from the beam splitter and directed toward the object, and a portion of light is reflected from the object and incident to a recording media (which is a film or glass plate covered with photographic emulsion). A second portion of laser light passes through the beam splitter and is reflected by a mirror toward the recording media such that the two laser light beams generate an interference pattern that is recorded on recording media. The photographic emulsion on the recording media is then developed by known photographic development process. The recorded hologram picture can be reconstructed by illuminating the developed film with the light beam at the appropriate incident angle, typically equal to the incident angle of the original reference light beam. The resulting reflected or transmitted light creates virtual or real image of the original object, depending on the setup of light beams during the recording. Some types of holograms may be observed (reconstructed) using conventional (non-laser) white light source. Other types of hologram may be observed only using laser light source.
However, the above process of creating hologram is only suitable for making a hologram of a static object, requiring an elaborate optical system well protected from vibration and external light. It also requires time for developing photographic emulsion on the recording media. It is therefore not suitable for recording images of moving objects.
Computers have been used to calculate interference patterns for holograms. However, constructing an operational high quality 3-dimensional holographic display is a technically difficult task. There are several attempts to design 3-dimensional holographic displays for moving images. A major difficulty in making holographic display arises from the fact that reproduction of a hologram interference pattern requires very high resolution of reconstruction media, on the order of a light wavelength xcex. In fact, the distance between the fringes of holographic pattern resulting from the interference of two laser beams is typically on the order of light wavelength xcex in most holographic systems. This is the main reason why DMDs and LCDs are not able to reproduce the desired interference pattern with necessary resolution because their pixel resolution is on the order of dozens of microns, which is much larger than xcex.
One attempt to construct an optical system for holographic display uses rotating or scanning mirrors to cover the required image area. As an example, U.S. Pat. No. 5,652,666 utilizes cylindrical lenses to magnify image from a DMD spatial light modulator in the vertical direction, and at the same time de-magnifies light in the horizontal direction to display one narrow vertical strip of the hologram image. A scanning mirror is then utilized to scan the image and display a composite consisting of many strips. A DMD device thus produces a hologram of the narrow strip of image, one at a time, and is illuminated by pulses of laser light synchronized with changing patterns and moving of the scanning mirror. In another example, U.S. Pat. No. 5,430,560 is for a device comprised of a light source, a rotating disk with sample holograms and a control unit that modulates light of light source in synchronization with the rotation of the hologram disk. In this and some other devices the 3-dimensional image is not generated as a whole, as hologram supposed to be, but by generating a small hologram area or even one pixel at a time and scanning the entire volume quick enough so that due to inertia of human vision it will be perceived as the whole image. There are many other schemes for holographic displays proposed using mechanical rotating or scanning elements and acoustical ultrasound elements to generate small parts of the hologram image.
One example of holographic display system proposed in U.S. patent application No. 20020176127 that provides system for displaying a real time moving three-dimensional hologram. The system provided in that invention comprises a computing device that generates hologram pattern (fringes) using a Fourier transform; a holographic pattern is displayed by DMD spatial light modulator that is illuminated by one portion of laser light. A reflected light is interfered with by another portion of laser light, firstly separated by beam splitter. Accordingly, the two portions of laser light interfere at a plane, which is allegedly produces a 3-dimensional holographic pattern.
U.S. patent application No. 20030016364 provides another example of a holographic recording and reproducing system. A system for hologram pattern acquisition disclosed in that application uses off-axis hologram recording setup, which is a variation of known xe2x80x9cMichelsonxe2x80x9d interferometer geometry. Geometrical arrangement of the beam splitter, reference beam and the object beam is chosen such that two light beams are combined on image recording device at a very small angle, which increases the distance between interference fringes and thus allows acquisition of fringe patterns by a conventional CCD device. It further utilizes an elaborated digital image processing such as extended Fourier transform (EFT) to generate the hologram pattern suitable for reconstruction of image of the original object. The image is further reconstructed by system comprising laser, beam splitter, mirrors, spatial light modulator, lenses and light sensitive photorefractive crystal. Light modulated by spatial light modulator induces a holographic diffraction grating pattern in the volume of photorefractive crystal. The hologram image is than reconstructed by illuminating reconstructing light beam onto said photorefractive crystal. The disclosed device therein uses a LCD-type of spatial light modulator. Like all of the holographic 3-dimensional display devices described above, this device suffers from the physical resolution limitation of existing types of spatial light modulators.
Another technical field utilizing spatial light modulators is holographic optical memory and optical information processing systems such as pattern recognition systems. It is known that optical holography can be used for pattern and image recognition as well as a means for optical data memory. One example of optical content-addressable memory system is disclosed in U.S. Pat. No. 6,205,107, for an optical storage system that includes a spatial light modulator, a plurality of hologram storage volumes and a correlation-plane detector. An example of optical information processing device is disclosed in U.S. Pat. No. 5,497,253, which is for a device designed for pattern recognition that utilizes a spatial light modulator, a volume hologram for correlation processing, a correlation detector, and nonlinear processing elements for modeling neuron behavior. That device also includes means for selective feedback from the output information-processing plane to the input spatial light modulator for modeling behavior of multi-layer neural network.
The performance, physical sizes and information density of holographic optical memory and pattern recognition systems depends greatly on the characteristics of spatial light modulator utilized for data input. Significant improvement in spatial light modulators resolution technology is required in order to achieve an economical ratio of storage space versus the device cost. Improvement in speed of spatial light modulator elements is required to achieve faster response time and higher data throughput.
Another emerging technical field where spatial light modulators are used is direct-write optical lithography. A typical lithographic system used in semiconductor manufacturing comprises a light source, an optical system such as lenses and collimators, and a photo mask. An image from the photo mask is demagnified (typically by a factor of xc2xc to {fraction (1/10)}) and projected upon a photosensitive polymer (photo-resist) layer on top of a substrate. A single photo mask is typically used to manufacture a number of identical chips. However, a spatial light modulator can replace the photo mask so that individual features on the substrate are determined dynamically by direct computer control. For example, U.S. Pat. No. 6,480,324 discloses such a system designed for producing a predetermined light pattern projected on the surface of the substrate under computer control. That system suggests using a LCD or DMD type of spatial light modulator. Accordingly, the direct write optical lithography is then used for synthesis of DNA arrays and polymer array synthesis, which particularly requires custom generated patterns for each element, and is not economically possible by conventional lithography. It would be desirable to extend same principle of direct write optical lithography for semiconductor manufacturing of small volume and experimental semiconductor devices, which do not require high volume production. However, existing types of spatial light modulators, such as DMDs or LCDs, do not provide sufficient resolution of pixels (elements) to produce patterns suitable for semiconductor manufacturing.
A further emerging technical field that utilizes spatial light modulators is adaptive optical systems. These systems are capable of optical compensation for distortions and aberration of a light beam in the atmosphere. Such systems are used, for example, in military applications for target tracking and aim-point selection. Some types of adaptive optical systems use deformable mirrors or lenses that are mechanically deformed under computer control in response to signals of waveform optical sensors. Other types of adaptive optics use optical phase conjugation and nonlinear optical devices. A third proposed type of adaptive optics known as xe2x80x9cholographic interactive trackingxe2x80x9d (HIT) is based on holographic principles. It includes the steps of illumination of a target by a laser beam and receiving the reflected light and recording the interference pattern of reflected light with a local oscillator beam. The resulting hologram pattern is digitally processed and transferred to a spatial light modulator where the hologram is generated. Laser light illuminating the spatial light modulator is directed toward the target and, due to the principles of conjugation, focuses on the target despite distortions and aberrations in the atmosphere. Examples of such systems are disclosed in U.S. Pat. No. 5,378,888, and No. 6,115,123. However, these types of systems are limited by the quality of holograms generated using existing DMD and LCD types of spatial light modulators and speed of updating the holographic image, which is a critical factor for tracking fast moving targets.
Chalcogenide materials (glasses) such as GeSbTe used in many electronic and optical devices. These materials have nonlinear optical properties, which make them useful for nonlinear optical fibers and filters. Chalcogenide materials are known to be able of reversible structural change between crystalline and amorphous state. These materials have highly nonlinear electrical conductance, which used in many devices. As an example, U.S. Pat. No. 5,757,446, is for a LCD light modulator (display) in which ovonic (chalcogenide) material is used for pixel switching (selection) element, which allows to apply voltage to the pixel located on the given intersection of address lines, instead of traditional switching elements such as a diode or transistor. However, the optical pixel element is a traditional liquid crystal and the pixels are subject to the same technological limitation of size, so that device is not suitable as high-resolution spatial light modulator.
Chalcogenide material is also used in data storage. A common example of storage is a rewritable optical disk. Such common optical disks as CD-RW and DVD-RAM routinely utilize physical and optical properties of chalcogenide materials. The disks incorporate a layer of reversible phase change material to allow the storage of information recorded optically through laser light. An example of an optical recording media that utilizes properties of chalcogenide materials is provided in U.S. Pat. No. 6,503,690, which discloses an optical recording media (such as a rewritable optical disk) with improved characteristics in repetitive recording and improved signal to noise ratio. The recording media is comprised of a recording layer containing Ge, Te and Sb and a diffusion-preventing layer in contact with the recording layer.
It has also been attempted to use phase-state changing material in semiconductor memory technology, and specifically for a non-volatile electronic memory device. These types of memory have been called xe2x80x9covonic unified memory.xe2x80x9d (OUM) The OUM devices use semiconductor structures with chalcogenide material as memory elements on top of the substrate. The memory cells change phase in response to appropriately timed pulses of electrical current, with 10 nanoseconds xe2x80x9cresetxe2x80x9d time and 50 nanoseconds xe2x80x9csetxe2x80x9d time, and the cells can survive (i.e. do not degrade material properties) after at least 1012 cycles of re-programming. Such memory elements are provided in U.S. Pat. No. 6,314,014, and 6,480,438.
Accordingly, a spatial light modulator having a high resolution of pixel elements can be utilized in many optical applications. It therefore to such an improved high-resolution, spatial light modulator that the present invention is directed.
The novel spatial light modulator device provided in the present invention utilizes the property of reversible phase change materials, and particularly alloys known as chalcogenide materials, to construct a high-resolution spatial light modulator. The inventive spatial light modulator can be used in a broad range of technical applications such as holographic displays, optical information storage and adaptive optics. The invention includes specific embodiments of optical systems using the high-resolution spatial light modulator, including a 3D holographic display, a holographic optical memory, a holographic pattern recognition/neural network system, a direct-write optical lithography, and an adaptive optics system for moving target illumination/tracking.
The spatial light modulator is comprised of a substrate, an array of optically reflective elements, and a control, preferably a circuit, to selectively control state of optical elements. The optical elements (pixels) use material that is capable of reversible structural phase change in response to heat or an electric current. One such material is a chalcogenide alloy such as GeSbTe that exhibits reversible transition from amorphous to crystalline phase in response to heat. The apparatus utilizes the difference in optical properties such as reflectivity or refraction of such material in amorphous and crystalline state. In one embodiment, the control circuit is comprised of column and row address selecting wires and selection elements, such as transistors or diodes, in the intersection of wires.
The inventive spatial light modulator comprises optically reflective elements that can generate pixels preferably with geometrical sizes lesser than 0.5 microns. This size is an order of magnitude smaller than common types of spatial light modulators, such as DMDs and LCDs, thus permitting may novel optical devices and applications. Such improved pixel resolution permits using spatial light modulator for displaying high quality holograms as well as many other novel optical applications.
In another aspect of the present invention, there is provided a novel spatial light modulator that permits a very fast update of image pattern. If so embodied, the change of state of pixel elements can occur in about 10-50 nanoseconds (ns), which is a few orders of magnitude improvement compared to DMD and LCD spatial light modulators.
In yet another aspect of the present invention, there is provided a device having the novel spatial light modulator and utilizes its unique physical principles to yield properties of memory, i.e. once the pattern (image) is recorded, that pattern remains physically stored in the pixel elements until another pattern (image) is recorded and the physical structure of the elements changed. Thus, the device does not require any image refresh signal and does not consume power in maintaining the same image or logical state. The power is consumed only at a time when a new image is recorded by changing state of pixel elements. For this reason, the spatial light modulator also is suitable as a display for portable low power electronic devices.
In yet a further aspect of the present invention, there is provided a device having the novel spatial light modulator that utilizes its physical principles to permit not only electrical but optical programming. The image (pattern) can be adjustable to either electrical signals or incident light. As such, the device provides a foundation for novel class of electro-optical elements for information processing, such as opto-electronic neural networks and fuzzy logic devices.
In yet another aspect of the present invention, there is provided a device that can utilize the novel spatial light modulator for novel types of adaptive optics applications such as illumination and tracking of moving targets.
In yet a further aspect of the present invention, there is provided a 3-Dimensional holographic display system architecture utilizing the high-resolution spatial light modulator. The holographic display system can be used for displaying computer-generated holograms and holographic images of objects acquired by other means, such as radiolocation, ultrasound scanning, or magnetic resonance imaging. Moreover, the system can be utilized to provide remote images holographically to a viewer.
In yet another aspect of the present invention, there is provided a holographic optical switching system for programmable redirecting light between multiple optical fibers or optically interconnected chips, which advantageously utilizes the reduced size and improved characteristics of the high-resolution spatial light modulator.
In yet a further aspect of the present invention, there is provided a direct-write optical lithography system utilizing the improved resolution and characteristics of the high-resolution spatial light modulator to project fine light patterns for lithography.
Other object, advantages, and features of the present invention will become apparent after review of the hereinafter set for Brief Description of the Drawings, Detailed Description of the Invention, and the claims.